1. Field of the Invention
The present invention is directed to a stress-meter for measuring stress in earth material such as rock or soil. The invention is further directed to a method and apparatus for installing the stress-meter.
2. Description of the Prior Art
It often becomes necessary to measure the stress of earth materials such as rock or soil. For example, the deformation measurements in soil or rock layers are of importance in fields such as geology, mining and oil prospecting. For this purpose, it is conventional to drill a borehole in the earth material whose stress is to be measured, and to insert sensors into the boreholes. Conventional sensors are photoelastic borehole plugs, to which are attached electrical resistant strain gages, load cells, flat jacks, vibrating wire gages or cylindrical jacks. The signals from the sensors are read out by suitable conventional equipment and subsequently processed into numeric data indicative of stress, by use of calibration factors obtained before insertion of the gages, or equations developed from an analytical method.
Examples of known systems for measuring stress in earth materials may be found in U.S. Pat. No. 3,349,610 to Noel and U.S. Pat. No. 2,927,459 to Farrington. However, known systems suffer the shortcoming of being expensive and being difficult to install and use. For example, in the Noel system the halves of a shell are separated by hydraulic piston elements located within the shell itself. It is therefore necessary to extend hydraulic lines far into the borehole, creating the danger of leakage and increasing the complexity of the system. In the Farrington arrangement, each section of the sounder is required to contain a motor and gearing for expanding probes associated with the strain gages. The requirement for such complex mechanical elements within the sounder itself also raises the problem of unreliability and complexity of construction.